This invention relates to a starter motor and more particularly to a starter motor having a pinion shift cylinder on which a pinion gear for engaging and driving an engine ring gear is provided and which is axially slidably supported on an output rotary shaft of the motor.
FIG. 1 illustrates one example of the conventional engine starter motor disclosed in Japanese Utility Model Laid-Open No. 61-6679.
In FIG. 1, the starter motor comprises a d.c. motor 1 for generating a rotational power for starting the engine and having an armature rotary shaft 2a, an output rotary shaft 2b integrally extended from the armature rotary shaft 2a of the d.c. motor 1, an over-running clutch 3 having a clutch inner member 3a and a clutch outer member 3b axially slidably fitted over the output rotary shaft 2b, a pinion 4 for engaging and driving an unillustrated ring gear of the engine, a pinion shift cylinder 5 for supporting the pinion 4 at its front end and connected at the rear end to the clutch inner member 3a of the over-running clutch 3, the pinion shift cylinder 5 being slidably fitted over the output rotary shaft 2b, and a housing 6 containing therein various components.
The pinion shift cylinder 5 is a hollow tubular member placed over the output rotary shaft of the d.c. motor 1 and rotatably and axially slidably supported by a bearing 7 fitted at its inner circumferential surface over the outer cylindrical surface 5a of the cylindrical body of the pinion shift cylinder 5. The outer cylindrical surface 5a of the pinion shift cylinder 5 is axially slidable relative to the inner surface of the bearing 7. In this context, the outer cylindrical surface 5a is a sliding support surface.
The starter motor also comprises a solenoid switch 8 having a plunger rod 8a and a shift lever 9. The shift lever 9 is connected at one end to the plunger row 8a and connected at the other end to the clutch outer member 3b of the over-running clutch 3. In order to rotatably support the pinion shift cylinder 5 on the output rotary shaft 2b, a bearing 10 is disposed between them and a stopper 11 is disposed at a position close to the front end of the output rotary shaft 2b. An end cap 12 is secured to the front end of the pinion shift cylinder 5 to cover the front end of the output rotary shaft 2b.
When the over-running clutch 3 is moved forward (to the right as seen in FIG. 1) by the shift lever 9, the pinion shift cylinder 5 is also moved forward along the output rotary shaft 2b, so that the pinion 4 on the pinion shift cylinder 5 is pushed out from an opening 13 of the housing 6 to engage the unillustrated engine ring gear. Just before the pinion 4 engages the engine ring gear, the d.c. motor 1 is energized through the solenoid switch 8 which also moves the shift lever 9. The rotation of the output rotary shaft 2b which is an extension of the armature rotary shaft 2a of the d.c. motor 1 is transmitted to the pinion shift cylinder 5 through helical splines 14 formed on the output rotary shaft 2b and the clutch outer member 3b and the clutch inner member 3a which is an integral part of the pinion shift cylinder 5.
With the conventional engine starter motor of this kind, since the pinion shift cylinder 5 must be supported in the housing so that it is axially slidable as well as rotatable, the inner circumferential surface of the bearing 7 is in contact with the outer cylindrical support surface 5a so that they are relatively slidable. Therefore, if any foreign matter sticks on the sliding surface 5a and caught between the sliding surface 5a and the inner surface of the bearing 7, the pinion shift cylinder 5 is prevented from smoothly moving around and along the output rotary shaft 2b. However, since the cylindrical support sliding surface 5a of the pinion shift cylinder 5 is only covered by the bearing 7 and other portion of the surface 5a is exposed within the interior of the housing. Therefore, foreign matters such as dust or the particles of the brush material from the electrical brushes of the d.c. motor 1 can easily fly onto the sliding surface 5a and stick on it, resulting in a jerky or irregular motion of the pinion shift cylinder 5 which significantly degrades the reliability of the starter motor.